293 PHYSIOGRAPHY. [cH. xvii. 



succession of changes in the physical geography of the 

 Thames' basin, has been brought about otherwise than 

 gradually. In all probability, no human inhabitant of the 

 region which has been the seat of these changes, would have 

 been more inclined to doubt that things always had been 

 as he saw them, that we are at present. And, although it 

 is demonstrated that the living population of the region has 

 undergone so thorough a change, that hardly a species which 

 inhabited sea, or land, in the Cretaceous period, is to be 

 found in, or near, the Thames basin of the present time ; it 

 is probable that, at any given epoch, the most observant 

 and accurate naturalists might have continued their obser- 

 vations for centuries together, without being able to discover 

 that the forms of animals and plants were other than fixed 

 and permanent. Twenty generations of day-flies, however 

 sharp their eyes, would fail to make out that the planet 

 Uranus changes its place in the heavens. 



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